Friends are as genetically similar as fourth cousins, a study by Yale and the
University of California has found

Blood is supposed to be thicker than water, but a new study suggests that friends are as genetically close to us as members of our own family.

Researchers who compared the DNA of unrelated friends were intrigued to find they had the same genetic similarity as fourth cousins, or people who share great-great-great grandparents.

The finding remained even after controlling for ethnic, cultural and geographical bias.

“We are somehow, among a myriad of possibilities, managing to select, as friends, the people who resemble our kin,” said Nicholas Christakis, professor of sociology, evolutionary biology, and medicine at Yale.

The researchers looked at 1.5 million markers of gene variation and found that friends are most similar in genes affecting the sense of smell and least compatible in genes controlling the immune system.

Recent studies have found that people appear to choose partners based on differences in immunity.

It has been suggested that forming social groups with others who are able to withstand different pathogens reduces the spread of disease and increases the chances of survival.

"Looking across the whole genome we find that, on average, we are genetically similar to our friends,” said co-author James Fowler, professor of medical genetics at the University of California.

“We have more DNA in common with the people we pick as friends than we do with strangers in the same population."

The academics believe that choosing friends who are genetically similar may convey ‘evolutionary advantages.’

"The first mutant to speak needed someone else to speak to,” added Prof Fowler, “The ability is useless if there's no one who shares it.”

However the mechanism for selecting friends or a mate based on genetic traits remains a mystery.

Previous research has suggested that pheromones in sweat carry important clues to genetic compatibility and are important to attraction. There are fears that modern medicines, like the pill, dampen down this natural selection process.

The so-called ‘smelly T-shirt experiment’ first performed by Swiss zoologist Claus Wedekin in 1994 found that women preferred the sweat of men who had immune systems the least like their own.

But Professor Dan Davis, of the University of Manchester, who recently published The Compatibility Gene, said attempts to repeat the experiment had shown mixed results.

“It is fascinating that there seems to be some underlying biology going on, but this area is very controversial and the evidence of this in humans is still extremely weak.

“It is certainly feasible that genetics is driving some kind of selection behaviour. We see it in animals but in humans there are so many other factors to consider that the findings have been inconclusive.”

Professor Fowler believes there may be a simpler explanation as to how people with similar genes meet and form friendships. People who like the scent of coffee, for example, may hang out at cafés more and so meet and befriend each other.

The researchers focused on 1,932 subjects and compared pairs of unrelated friends against pairs of unrelated strangers.

They found that the genes that were more similar between friends seem to be evolving faster than other genes.

Profs Fowler and Christakis say this may help to explain why human evolution appears to have speeded up over the last 30,000 years, and they suggest that the social environment itself is an evolutionary force.

“It seems that our fitness depends not only on our own genetic constitutions, but also on the genetic constitutions of our friends,” added Prof Christakis.

The team has even developed a ‘friendship score’ which rates how likely two people will be friends based on their DNA.

The findings were published in the journal Proceedings of the National Academy of Sciences.